Circulating tumor markers are being studied to aid in the detection and diagnosis and monitoring the course of patients with various neoplastic diseases. Sensitive quantitation of alpha-feroprotein (AFP) by radio-immunoassay has been shown to be of considerable value for following therapy in patients with hepatocellular carcinoma (HC) and testicular cancer of germ cell origin where quantitation of human chorionic gonadotropin (HCG) also serves as an indicator of tumor activity. The usefulness of AFP in non-malignant diseases such as congenital neural tube defects and infectious hepatitis is also being studied. Experimental studies in primates with chemically-induced HC allow the demonstration of the usefulness of AFP for early detection of tumor and also to calculate the synthetic (rate) and catabolic rate of AFP to derive the total body load of tumors cells. Progress continues on the isolation of a lung tumor associated antigen (LTAA) using physicochemical and immunochemical technqiues. Isolation will be monitored by radial immunodiffusion, leukocyte migration inhibition (LMI) and by delayed hypersensitivity skin testing. After antigen purification an RIA will be developed for the assay of patient serum. Similiar studies are being performed using a murine virus-induced leukemia (MBL-2) as a model system and inhibition of Cr51 release cytotoxicity by antibody and complement as the antigen detecting system.